# std::vector<T,Allocator>::emplace_back

From cppreference.com
< cpp‎ | container‎ | vector

template< class... Args > void emplace_back( Args&&... args ); |  | (since C++11) (until C++17)
template< class... Args > reference emplace_back( Args&&... args ); |  | (since C++17) (constexpr since C++20)

Appends a new element to the end of the container. The element is constructed through std::allocator_traits::construct, which typically uses placement-new to construct the element in-place at the location provided by the container. The arguments args... are forwarded to the constructor as std::forward<Args>(args)....

If after the operation the new `size()` is greater than old `capacity()` a reallocation takes place, in which case all iterators (including the `end()` iterator) and all references to the elements are invalidated. Otherwise only the `end()` iterator is invalidated.

### Parameters

args | - | arguments to forward to the constructor of the element
Type requirements
- `T (the container's element type)` must meet the requirements of MoveInsertable and EmplaceConstructible.

### Return value

(none) | (until C++17)
A reference to the inserted element. | (since C++17)

### Complexity

Amortized constant.

### Exceptions

If an exception is thrown for any reason, this function has no effect (strong exception safety guarantee). If the move constructor of `T` is not noexcept and is not CopyInsertable into *this, `vector` will use the throwing move constructor. If it throws, the guarantee is waived and the effects are unspecified.

### Notes

Since reallocation may take place, `emplace_back` requires the element type to be MoveInsertable for vectors.

### Example

The following code uses `emplace_back` to append an object of type `President` to a std::vector. It demonstrates how `emplace_back` forwards parameters to the `President` constructor and shows how using `emplace_back` avoids the extra copy or move operation required when using push_back.

Run this code

    #include <vector>
    #include <cassert>
    #include <iostream>
    #include <string>

    struct President
    {
        std::string name;
        std::string country;
        int year;

        President(std::string p_name, std::string p_country, int p_year)
            : name(std::move(p_name)), country(std::move(p_country)), year(p_year)
        {
            std::cout << "I am being constructed.\n";
        }

        President(President&& other)
            : name(std::move(other.name)), country(std::move(other.country)), year(other.year)
        {
            std::cout << "I am being moved.\n";
        }

        President& operator=(const President& other) = default;
    };

    int main()
    {
        std::vector<President> elections;
        std::cout << "emplace_back:\n";
        auto& ref = elections.emplace_back("Nelson Mandela", "South Africa", 1994);
        assert(ref.year == 1994 && "uses a reference to the created object (C++17)");

        std::vector<President> reElections;
        std::cout << "\npush_back:\n";
        reElections.push_back(President("Franklin Delano Roosevelt", "the USA", 1936));

        std::cout << "\nContents:\n";
        for (President const& president: elections)
            std::cout << president.name << " was elected president of "
                      << president.country << " in " << president.year << ".\n";

        for (President const& president: reElections)
            std::cout << president.name << " was re-elected president of "
                      << president.country << " in " << president.year << ".\n";
    }

Output:

    emplace_back:
    I am being constructed.

    push_back:
    I am being constructed.
    I am being moved.

    Contents:
    Nelson Mandela was elected president of South Africa in 1994.
    Franklin Delano Roosevelt was re-elected president of the USA in 1936.

### See also

push_back | adds an element to the end (public member function)
emplace (C++11) | constructs element in-place (public member function)

Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/container/vector/emplace_back&oldid=124989"